1. Field of the Invention
The invention generally relates to a take-off control apparatus and method for a vehicle equipped with an automatic transmission. More particularly, the invention relates to a take-off control apparatus and method for a vehicle, which executes a neutral control.
2. Description of the Related Art
An automatic transmission mounted in a vehicle is connected to an engine via a fluid coupling such as a torque converter provided in the automatic transmission, and includes a gear change mechanism that provides a plurality of power transmission paths. This automatic transmission is structured so as to automatically switch the power transmission path, i.e., the gear ratio (drive speed), based on the accelerator opening amount and the vehicle speed, for example. A vehicle having an automatic transmission is typically provided with a shift lever which is operated by a driver into any one of various shift positions (such as REVERSE, NEUTRAL, DRIVE). Automatic shift control is performed when the shift lever is in one of these shift positions (usually in the drive position or forward-drive range).
When a vehicle having this type of automatic transmission is stopped with the shift lever in a position corresponding to a forward-drive range, such as DRIVE, driving force from the idling engine is transmitted to the transmission via the torque converter and then to the wheels, resulting in a phenomenon known as “creeping.” Creeping is extremely useful under certain conditions. For example, it facilitates smooth take-off of a vehicle when starting from a standstill on an incline. When the driver wants a vehicle that is stationary on a flat road to remain in one spot, however, creeping is unnecessary and must be suppressed by operating the brake. That is, the brake is used to suppress the creeping force from the engine and the fuel efficiency of the engine decreases by a corresponding amount.
To improve fuel efficiency, therefore, it has been proposed to put a transmission that is in the forward-drive range into a neutral state, including a semi-neutral state, that resembles NEUTRAL while still in the forward-drive range and the vehicle is being held stationary by the brake pedal being depressed so as to operate the brake and the accelerator being almost completely closed.
Many technologies for this kind of so-called neutral control and controlling a vehicle during the transition from a stationary state to a take-off state are known.
JP(A) 2000-304127 discloses a control apparatus for an automatic transmission, which is capable to preventing a variety of problems caused by delayed application of a forward clutch when the neutral control is cancelled. This kind of control apparatus includes i) slip state controlling means for controlling a specific friction apply element used to establish a predetermined speed into a predetermined slip state when a predetermined condition to start the control has been satisfied, ii) control canceling means for returning the friction apply element to the applied state by reducing the input rotational speed of the friction apply element in accordance with a predetermined rate of change when a predetermined condition to cancel the control has been satisfied based on the intention of a driver to take-off, and iii) rate of change correcting means for increasing the rate of change of the input rotational speed when at least one of a) an increase in engine torque and b) backing up of the vehicle has been detected during restoration of the friction apply element to the applied state by the control canceling means.
According to this control apparatus for an automatic transmission, the input rotational speed is quickly reduced in accordance with the increase-corrected rate of change when the engine torque suddenly increases due to, for example, a sudden operation of the accelerator. As a result, the friction apply element is returned to the applied state before the engine starts to race, thus enabling a smooth take-off to be achieved. Further, in a case in which a vehicle stopped on an incline starts to slide backward when the driver releases the brake pedal when starting to take off, the friction apply element applies quickly enough to prevent the vehicle from sliding backward unintentionally.
JP(A) 2000-304128 discloses a control apparatus for an automatic transmission, which is capable of preventing a control over-correction due to a fluctuation in line pressure when applying a forward clutch as neutral control is cancelled, and therefore capable of preventing shock from being generated by sudden application of the forward clutch. This control apparatus includes i) slip state controlling means for controlling a specific friction apply element used to establish a predetermined speed into a predetermined slip state when a predetermined condition to start the control has been satisfied, ii) apply controlling means for returning the friction apply element to the applied state when a predetermined condition to cancel the control has been satisfied based on the intention of a driver to take-off, iii) apply force increasing means for increasing the apply force on the friction apply element when an increase in engine torque has been detected while the friction apply element is being returned to the applied state by the apply controlling means, iv) line pressure detecting means for detecting an increase or decrease in line pressure in the automatic transmission, and v) over-correction preventing means for suppressing an increase in the apply force by the apply force increasing means in accordance with an increase in the line pressure of the automatic transmission.
Accordingly, this control apparatus for an automatic transmission suppresses an increase-correction in the apply force, which is executed in response to an increase in engine torque, from being executed in accordance with an increase in line pressure in the automatic transmission when the control keeping the friction apply element in a slip state is cancelled. As a result, over-correction of the apply force caused by a fluctuation in the line pressure can be prevented, and therefore shock can be prevented from being generated by sudden application of the friction apply element.
Despite these advantages, however, the control apparatuses in the foregoing publications have the following problems.
The control apparatus for an automatic transmission that is disclosed in JP(A) 2000-304127 is simply an apparatus which increases the target rate of change of the turbine speed in order to return the forward clutch to the applied state when an accelerator operation is performed when the neutral control is cancelled. The control apparatus for an automatic transmission that is disclosed in JP(A) 2000-304128 is simply an apparatus which i) increases a duty ratio of a solenoid to suppress the forward clutch from slipping when an accelerator operation is performed when the neutral control is cancelled, and ii) sets an engine speed correction amount in accordance with an increase in an ATF line pressure and decreases the duty ratio of the solenoid by that correction amount when an engine speed at that time is within a predetermined range.
In either case, good controllability is unable to be achieved in cases such as when the engine speed fluctuates and when a target speed ratio (the speed ratio is the rotational speed ratio of the torque converter; speed ratio=turbine speed/engine speed) deviates from a present speed ratio, both of which occur when the neutral control is cancelled.
More specifically, as the engine speed fluctuates, so does the torque transmitted to the drive train. An increase or decrease in the torque results in excessive or insufficient hydraulic pressure. Furthermore, despite the fact that control is performed to reduce the turbine speed during cancellation of the neutral control, as the engine speed increases (or decreases), force is also generated that tries to increase (or decrease) the turbine speed.
Also, the apply hydraulic pressure of the forward clutch changes when the present speed ratio deviates from the target speed ratio. A constant standby pressure during cancellation of the neutral control is calculated by adding a predetermined hydraulic pressure to a base pressure, which is an apply hydraulic pressure of a forward clutch just before the neutral control is cancelled. As a result, when the apply hydraulic pressure of the forward clutch fluctuates, the absolute value of the constant standby pressure changes, resulting in a constant standby pressure that is either excessive or insufficient.
If the hydraulic pressure (constant standby pressure) is too high when the neutral control is cancelled, the forward clutch applies suddenly which generates a shock. If the hydraulic pressure (constant standby pressure) is too low when the neutral control is cancelled, the forward clutch is unable to be applied sufficiently. As a result, the hydraulic pressure must be forcibly increased, which also generates a shock.
Moreover, when the engine speed fluctuates when the neutral control is cancelled or when the present speed ratio deviates from the target speed ratio when the neutral control is cancelled, the turbine speed behaves different than normally. If learning control is executed to learn the apply hydraulic pressure so that the amount of change in the turbine speed becomes equal to a desirable amount of change (i.e., equal a desirable ratio of change over time) in this case, the learning is erroneous.